Nuclear reactors which are cooled by means of pressurized water comprise a core which is constituted by fuel assemblies, each fuel assembly itself being constituted by a cluster of fuel rods which are retained in a framework of the fuel assembly so that the rods, having a cylindrical form, have their axes mutually parallel.
Each of the fuel rods comprise a rectilinear cylindrical tubular cladding that has an axial length that is much greater than the diameter of the cross-section. Generally, the fuel rods have a length greater than 4 m and a diameter in the order of 10 mm. The fuel rods comprise, at the inner side of the tubular cladding a column of nuclear fuel pellets which are stacked one on top of the other in the axial direction of the cladding which is closed, in a tight manner, at the axial ends thereof by means of plugs. In its operating position within a fuel assembly inside the core of the nuclear reactor, the fuel rod is arranged with the axis thereof vertical so that a first closure plug of the rod is at the lower portion thereof and constitutes the lower plug and the second plug which is located at the upper portion of the fuel rod in the operating position, constitutes the upper plug.
The column of fuel pellets rests with a first lower end on an end surface of a portion of the lower plug that is engaged inside the cladding of the fuel rod. The column of fuel pellets is retained inside the fuel rod by means of a helical compression spring which is interposed between a second upper end of the column of fuel pellets and an end surface of an inner portion of the upper closure plug of the rod.
In order to perform the filling of the rods during their construction, it is possible to close one of the ends of the tubular cladding of the rod, for example, the first lower end, by engaging the lower plug in one end of the tubular cladding and fixing it to the cladding by means of welding. The fuel pellets which are stacked one on top of the other in the form of a column whose lower pellet rests on the lower plug of the rod are then introduced inside the cladding. The spring is then positioned and the upper plug is fixed, and the gases inside the cladding are then discharged, generally by means of a channel which is provided through the upper plug, an inert gas, for example, pressurized helium, is introduced inside the cladding of the rod, and the rod is closed in a tight manner by welding a seal weld to the end of the channel which extends through the upper plug.
Inside the rod, the fuel pellets are in contact with pressurized helium which reduces or prevents oxidiation of the pellets. The inert gas fills all the portions of the cladding which are not occupied by the column of fuel pellets and the retention spring. In particular, pressurized helium fills the space which is provided between the upper end of the column of fuel pellets and the end of the inner portion of the upper plug in which the compression spring is arranged which retains the column of fuel pellets. This space constitutes a gas collection arrangement or plenum in which the gases discharged by the fuel pellets collect when the fuel rod is used in the core of a nuclear reactor. Some gases may be formed in particular by fission reactions of specific elements which are contained in the fuel pellets. The pressure at the inner side of the cladding of the rod therefore has a tendency to increase due to the gas being released by the column of fuel pellets. It is necessary to provide a plenum which has sufficient volume to prevent too strong an internal pressurization of the rod during operation.
With fuel rods that have pellets comprising a mixed oxide of uranium and plutonium or a mixture of oxides of uranium and plutonium (MOX fuel), the volume of the plenum between the upper portion of the column of fuel pellets and the end surface of the inner portion of the upper plug of the fuel rods is generally insufficient to completely ensure the tightness of the rod during operation. It is not possible to significantly increase the length of this space and therefore the length of the spring, which is not desirable in any case in so far as this extension of the upper plenum and the spring would cause the design of the rods to be modified and would require specific production of the constituent elements of the MOX fuel rods.
A conventional method for increasing the volume for expansion of the gases inside MOX fuel rods involves interposing, between the upper end surface of the lower plug inside the cladding and the first lower end of the column of fuel pellets, a cross-member or shim of stainless steel or zirconium alloy which is constituted by a tube that has a thick wall. In this manner, the MOX fuel rod comprises both an upper plenum and a lower plenum which extend between the lower end of the fuel rod and the upper end of the inner portion of the lower plug, which has the advantage of improving the diffusion of pellet gases towards the lower plenum which complements the upper plenum of the rod, balancing the hydraulic and thermal behaviour of the rod and reducing the operating temperature of the lower portion of the fuel rod.
However, this embodiment of MOX fuel rods has the disadvantage of requiring the production, the storage and the positioning of tubular shims during the industrial production of the fuel rods. Controlling the production is made more complex and controls must be performed if it is desirable to prevent any risk of a production fault, such as the omission of a shim, during the production of a rod.
It has also been proposed, for example, in EP-0 169 422 and JP 298 792 that there be provided, in lower or upper plugs of fuel rods, recesses which increase the expansion space for gases at the ends of the rod. Recesses of this type which extend both in the portion of the plug which is introduced into the cladding and in the portion of the plug outside the cladding lead to a significant modification of the design of the plugs. The increase in the expansion space due to the recesses which are machined in the plugs can be combined with the use of hollow cross-pieces inside the cladding between a plug and the column of fuel pellets.